Add group-class withdrawal deadline and kind-aware offering form

Group classes now carry an optional per-class withdrawal deadline. Up to
that day a student may withdraw themselves from the class; the withdrawal
frees the seat and voids any pending payment but never issues an account
credit. After the deadline self-withdrawal closes and a studio admin must
withdraw the student by hand (the admin path is never subject to the
deadline). A blank deadline keeps self-withdrawal open indefinitely.

Also make the Add/Edit Offering form show only the fields relevant to the
selected kind: group settings for group classes, weekly reservation for
private lessons. Progressive enhancement — without JS every field renders.

- New nullable us_offerings.withdrawal_deadline column; Offering model gains
  $withdrawalDeadline + isWithdrawalOpen().
- New student endpoint POST /enrollments/{id}/withdraw, gated by the deadline
  (403 withdrawal_closed), ownership-checked, idempotent.
- Front-end group-class page shows a Withdraw button while open.
- No USC_VERSION bump: 1.2.0 is unreleased and accumulates schema changes
  under its section, matching the scheduled-billing and credit features.

Tests: composer test (596), composer lint, composer cs all pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/Offering/Offering.php

@@ -69,6 +69,7 @@ class Offering {
 		public readonly ?string $termEnd = null,
 		public readonly ?string $classTime = null,
 		public readonly ?string $enrollmentDeadline = null,
+		public readonly ?string $withdrawalDeadline = null,
 		public readonly ?string $scheduleNote = null,
 		public readonly ?string $etransferEmail = null,
 		public readonly ?int $cancellationCutoffHours = null,
@@ -114,6 +115,19 @@ class Offering {
 		return null === $deadline || $today <= $deadline;
 	}
 
+	/**
+	 * Whether a student may still withdraw themselves from this group class on
+	 * `$today` (a `Y-m-d` date). Withdrawal stays open through the end of the
+	 * deadline day. Unlike the enrolment deadline there is no implicit default: a
+	 * class with no withdrawal deadline set stays open to withdrawal for its whole
+	 * life, so the instructor must set a date to lock students in. A withdrawal
+	 * made while open never issues an account credit — it only frees the seat and
+	 * voids any still-pending payment.
+	 */
+	public function isWithdrawalOpen( string $today ): bool {
+		return null === $this->withdrawalDeadline || $today <= $this->withdrawalDeadline;
+	}
+
 	/**
 	 * Normalise a submitted term date to canonical `Y-m-d`, or null when it is
 	 * not a real calendar date. Round-trips through DateTimeImmutable so
@@ -214,6 +228,7 @@ class Offering {
 			termEnd:         Val::stringOrNull( $row->term_end ),
 			classTime:       Val::stringOrNull( $row->class_time ?? null ),
 			enrollmentDeadline: Val::stringOrNull( $row->enrollment_deadline ?? null ),
+			withdrawalDeadline: Val::stringOrNull( $row->withdrawal_deadline ?? null ),
 			scheduleNote:    Val::stringOrNull( $row->schedule_note ),
 			etransferEmail:  Val::stringOrNull( $row->etransfer_email ),
 			cancellationCutoffHours: Val::intOrNull( $row->cancellation_cutoff_hours ),
@@ -249,6 +264,7 @@ class Offering {
 			'term_end'                  => $this->termEnd,
 			'class_time'                => $this->classTime,
 			'enrollment_deadline'       => $this->enrollmentDeadline,
+			'withdrawal_deadline'       => $this->withdrawalDeadline,
 			'schedule_note'             => $this->scheduleNote,
 			'cancellation_cutoff_hours' => $this->cancellationCutoffHours,
 			'access_mode'               => $this->accessMode,